1. Field of the Invention
The present invention relates to an electric power steering apparatus assisting a steering by a motor, and more particularly to an electric power steering apparatus in which a motor is PWM controlled.
2. Description of the Related Art
As an electric power steering apparatus, there has been generally known a structure in which a torque sensor detecting a steering torque is attached to an input shaft to which a steering wheel is fixed, and a steering force is reduced by generating an assist torque in correspondence to the steering torque detected by the torque sensor by an electric motor. In the electric power steering apparatus mentioned above, an applied voltage of the electric motor is generally adjusted by changing a duty ratio with a pulse width modulation (PWM).
FIG. 11A shows an FET circuit adjusting the applied voltage to the motor by a switching. A resistance R1 for slow switching the FET is connected to a gate side of the FET, a noise generation due to a rectangular wave shaped pulse generation is prevented by slow switching.
In FIG. 11B, reference symbol a denotes a control signal of the FET circuit, reference symbol b denotes an applied voltage to the gate of the FET, and reference symbol c denotes an on-off of the FET. After the control signal a of the FET is changed to a high level (a timing t1), the applied voltage b to the gate of the FET reaches a high level with a time lag by a time constant circuit (a timing t2), and the FET is turned on in accordance with this. In the same manner, after the control signal a of the FET circuit is changed to a low level (a timing t3), the applied voltage b to the gate of the FET reaches a low level with a time lag by the time constant circuit (a timing t4), and the FET is turned off in accordance with this.
In this case, the prior art discloses a technique of applying a voltage dither signal to the motor at a time of a low duty ratio in order to eliminate a dead zone in the low duty ratio.
[Prior Art] JP 2003-11834 A is incorporated herein by reference.
However, in the slow switching FET circuit shown in FIG. 11A, there is generated a problem that the FET can not be turned on at a time of the low duty ratio. In other words, at a time of the low duty ratio as shown in FIG. 11C, before the control signal a of the FET circuit is changed to the high level (the timing t1), and the applied voltage b to the gate of the FET reaches the high level with the time lag by the time constant circuit, the control signal a of the FET circuit is changed to the low level (the timing t3) and the FET can not be turned on. FIG. 12A is a graph showing a relation between a duty ratio and a current of a prior art. A state in which the current does not flow, that is, the dead zone is generated near a portion in which the duty ratio is zero. In accordance with a method of eliminating the dead zone mentioned above, it is possible to raise the duty ratio at a time of the low duty ratio, however, in this method, since an operation varies on the basis of a variation of the FET, it is possible to completely eliminate the dead zone only in the case that a simple substance modulation per each of elements (a map preparation per each of elements) doing away the variation of the FET is executed.
In the prior art, the dead zone in the low duty ratio is adjusted by applying the voltage dither signal to the motor at a time of the low duty ratio. However, in accordance with the structure mentioned above, it is impossible to apply to the FET circuit executing the slow switching as shown in FIG. 11A, and it is impossible to prevent the noise from being generated.